1. Field of the Invention
This invention relates to a two-way information transmission system such as video on-demand system and information on-demand system using a communication network such as cable television network.
2. Description of Related Art
Not only usual analog broadcasting service but also video on-demand, home shopping, and television game service utilizing a two-way cable television network have been recently planned.
One method for realizing such services is a method in which a digital information transmission band is provided, for example, in the transmission band of the television network separately from the band for analog broadcasting service. In detail, for example in the case of video on-demand system, a user transmits a demand signal (transmission request signal) of the video on-demand to a cable television station where a video program storing sending system called video server is installed using a portion of the digital information transmission band. Upon receiving the demand signal from the user, the cable television station takes out the program requested by the user from the video server, and provides it to the requesting user in real time using a band other than that for the above-mentioned demand signal in the above-mentioned digital information transmission band.
One of such video on-demand systems is introduced in a journal "Nikkei Electronics" issued May 23, 1994 on pages 82 to 89.
This example involves a hybrid network system comprising combined optical fiber and coaxial cable, the optical fiber connects from the cable television station to an optical fiber node, the optical fiber node performs photoelectric conversion, and the coaxial cable connects from the optical fiber node to a subscriber terminal called as set top box installed in the user home. Assuming that 16 optical fiber nodes are provided and the maximum number of subscribers connectable to a coaxial cable is 500, then one cable television station can supply to 8000 subscribers.
In assignment of transmission band of the cable television, This video on-demand system assigns the band range from 50 MHz to 723 MHz to a transmission band for going-down from the cable television station to a subscriber terminal of a subscriber, and the band range from 900 MHz to 1 GHz to a transmission band for going-up from a subscriber terminal to the cable television station, further, a transmission band between 50 MHz to 450 MHz out of the going-down transmission band is used for the existing analog broadcasting service and a transmission band between 500 MHz to 708 MHz is used for digital transmission band. and the residual transmission band ranging from 450 MHz to 500 MHz is spare as shown in FIG. 1
At most 15 digital transmission channels with a band width of 12 MHz can be provided in the digital information transmission band ranging from 500 MHz to 708 MHz. Assuming that the transmission speed per one channel is 45 M bits/sec and, for example, the coding speed of video data is 4 M bits/sec, video data can be supplied to about 10 subscribers by way of a channel with the transmission speed of 45 M bits/sec in real time.
Assuming that the proportion of requesting subscribers for demand to 500 subscribers is 25% in the busy time called as prime time, it is required to supply to 125 subscribers, and one channel can supply to 10 subscribers, therefore 15 channels can supply to 150 subscribers and thus the requirement is satisfied.
In the case of this example, the control channel is assigned near the frequency of 500 MHz, and the going-down channel to be used for supplying a video program data desired by a user through demand is informed to the set top box using this control channel. The set top box fetches only a desired going-down data in the cable television station based on the data of the control channel, and functions to display it on a television. The transmission speed of the control channel is 1.5 M bits/sec, and 45 channels and 1 channel are for going-up and for going down respectively.
FIG. 2 shows how the transmission capacity of going-down line is allocated to respective subscribers, the left half of the figure shows so-called prime time and the right half shows the midnight time zone.
In FIG. 2, the top half shows the transmission channels for analog television broadcasting service, each long rectangle represents one analog television program PGa. In this case, 50 channels of analog television broadcasting service are possible to be transmitted if one channel has a band width of 6 MHz. Each subscriber can select and view a desired program from programs being broadcasted by way of analog channels any time.
The bottom half of FIG. 2 shows the digital information channels for video on-demand, an arrow Tdm represents a demand generating time point from a user, and a long rectangle represents a one digital video program PGd. In response to a demand from a user, one channel is occupied by the user for a time of the service.
In FIG. 2, rating is high for the analog broadcasting program and also demand generation rating is high for the digital video program in prime time, on the other hand, rating is low for the analog broadcasting program and also demand generation rating is low for the digital video program in midnight.
As described hereinabove, in the conventional video on-demand system, video programs are supplied in real time to all the demands. Therefore, in the time zone like so-called prime time when many demands are requested from subscribers, many programs should be transmitted at a time as shown in the left side of FIG. 2.
For construction of a video on-demand system, the network capacity and transmission system are deployed so that programs are supplied to subscriber homes as soon as possible in real time in the busiest time zone when many demands are requested.
To cope with such situation, the conventional system is involved in a problem that 1 the network capacity should be large, and 2 the scale of video server should be large in order to supply many programs simultaneously (for example, assuming the number of subscribers is 8000 and 25% of these subscribers request their demand in prime time, the simultaneous transmission stream are 2000 streams). The capacity investment for enlargement of an existing facility is required, and an expensive large scale video server should be introduced, these expenditure can render the business substantially unprofitable.
As described herein above, many demands are generated very frequently in prime time, on the other hand, a few demand is generated sporadically in the time zone of early morning and midnight, the problem is inefficient use of the network facility having a large capacity deployed so as to match with busy demand in prime time, the capacity utilization ratio is low.
Further, the change of a cable television facility from existing one-way system to the above-mentioned two-way system requires the large scale replacement of infrastructure due to the above-mentioned problem, the expenditure for such investment is also very heavy and a problem.
It is the object of the present invention to provide a two-way information transmission system for solving problems described herein above.